When Sirius was a kid his mum always made him get haircuts in a ‘smart’ style he never liked .
it was still long enough on top to get knotted .
So every day his mum would yank a brush through his hair till his eyes watered and he felt like he’d been scalped.
He honestly thought a purpose of a brush was to hurt him. And he just assumed other people did it as a weird fashion thing. Like muggle lip injections
So first thing his did at hogworts was to grow his hair out and brush his hair as little as he could.
Third year came around and he’d settled on growing it out to his shoulder and he was happy with his look.
It wasn’t as messy as James’s but not the neet ponytail Remus wore.
He always joked he had the best sex hair.
One day he had put his hair up in a ponytail for herboligy and when he took it out it got stuck.
Panicking he would have to cut his hair he ran to Remus who had similar length hair as him .
Remus assured him they wouldn’t need to cut it off .
He sat him down getting out a Combe saying he could brush it out.
Sirius jumps away saying that maybe cutting it wouldn’t be so bad.
Remus manages to cokes him back saying if it hurts he’ll stop.
True to his word he slowly works the knots out and freeing the elastic .
Once he was finished he could work the brush all the way through his hair without snagging once.
Sirius found he actually enjoyed this new way of getting his hair brushed.
Remus found he didn’t mind being able to play with Sirius’ hair. It was thicker then his own mousy hair .
Soon enough it became a ritual of theirs.
Every other day in the common room Remus would be sat on the sofa near the fire and Sirius would come down with a brush and sit in the floor in-between his legs.
And Remus would brush out and play with his hair.
It also became common that Sirius would fall asleep Wilst having his hair messed with.
Quite often james would come in to see Sirius asleep against Remus leg resting his head in his knee
James teases him saying it was a dog thing
Sirius enjoyed it too much to care